The invention relates to a thermal spraying material, to the use of a material for the coating of a surface of a workpiece by means of a thermal spraying method, to a thermally sprayed coating, to a thermal spraying method and also to a thermally coated workpiece.
The coating of surfaces of the most diverse workpieces has an almost incalculable number of uses in industrial technology and a correspondingly high economic significance. In this connection coatings can be advantageously applied to the most diverse substrates for very different reasons. Wear protection coatings on mechanically heavily loaded parts such as, for example, on running surfaces of cylinders or piston rings of combustion engines or compressors, play an important role for example. In addition to wear resistance further demands are made on these parts, such as good sliding characteristics, i.e. good tribological characteristics or also excellent dry operation characteristics. Different thermal spraying methods have in particular proved to be excellent for such requirements and similar ones, above all the known plasma spray methods.
Coatings manufactured by arc vaporization, PVD or CVD processes have been used successfully for the production of hard layers on highly loaded tools, primarily chip-forming tools such as milling cutters, drills, etc. However, precisely the application of the last-named processes is very widespread in completely different fields, for example for the coating of jewelry or clock housings or for the application of protective coatings or simply for the embellishment of basic commodities.
Other methods, such as for example gas nitriding, are well-established methods which are of great significance in corrosion protection, among other things.
In this connection the coating of workpieces with very large surface areas, such as metal sheets for example, which have to be protected against corrosion among other things, is fundamentally problematic. Metal sheets of this kind or other substrates are, for example, provided on rolls of a substantial width of up to a few meters and in a length of up to several hundred meters or even more.
An established technique for the coating of sheets of this kind is galvanic or electrolyte deposition, for example. Thus, it is known, for example, to provide large steel sheets with a corrosion protection layer made of Zn and Mg. In a typical known process a 1 μm to 30 μm thick layer made of pure zinc (Zn) is applied in a first step electrolytically or galvanically onto a metal sheet which can be, for example, made of steel, of aluminum, of another metal or of a metal alloy. The surface of this first layer is then subjected to a cleaning process by means of ultrasound and/or PVD sputtering for example. After this a thin layer approximately 0.1 μm to 0.5 μm thick, made of pure magnesium (Mg), is applied to the first zinc layer by means of a PVD method. Finally, a heat treatment of the workpiece takes place for example for 10 minutes to 3 hours, at 200° C. to 550° C. for example, in very special cases up to 650° C., through which diffusion processes are initiated, so that MgZn2 phases can form on the surface of the originally pure Mg layer, through which an improved protection against corrosion is attained.
The workpieces treated in this way have clearly improved corrosion characteristics in comparison to workpieces with pure zinc layers and can also be worked better because the combination layers as a whole have a reduced thickness. However, the above-described four-stage coating process is extraordinarily time-consuming and above all requires the combination of completely different methods, so that not only the carrying out of the coating process as such but also machinery cost is enormous, so that the costs for the manufacture of these corrosion protection layers are basically unreasonably high.
For these reasons alternatives have been sought for a long time, with thermal spraying in its different variants basically coming into question, primarily because thermal spraying has long been established in the series production of individual parts and industrial series production. The most usual thermal spraying methods, which are also used in particular in series production for the coating of the surfaces of substrates in large numbers, are, for example, flame spraying with a spraying powder or a spraying wire, arc spraying, high velocity flame spraying (HVOF), flame detonation spraying or plasma spraying. The above-named list of thermal spraying methods is certainly not exhaustive. On the contrary the person averagely skilled in the art is familiar with a large number of variations of the listed methods, and also further methods, for example special methods such as flame spraying welding. The so-called “cold gas spraying” also has to be mentioned in this connection. Although, strictly speaking, it is not to be counted as one of the thermal spraying methods, within the context of this application the known “cold gas spraying method” (cold spraying) is also understood to be covered by the term “thermal spraying method” in addition to all known spraying methods.
In this connection thermal spraying has opened up large areas of use. One can certainly state that thermal spraying as a surface coating technique is the coating technique which probably has the largest scope of use. A delimitation of the areas of use of the spraying methods listed above does not appear necessarily sensible in this connection because the areas of use can overlap with each other.
It was a great problem for a long time in this connection to provide large surface areas in sufficient uniformity, in particular with thin layers with thickness in the micrometer range by means of thermal spraying methods. The low pressure thermal method (“LPPS method”) suggested in EP 0776 594 B1 has brought about a breakthrough here, which, using a broad/wide plasma beam, allows the manufacture of uniform coatings on large surfaces, on metal sheets for example. On the one hand this is achieved by means of the geometric design of the spray gun, with it moreover being important that a substantial pressure difference prevails between the inside and the outside of the spray gun. The workpiece, or at least the surface area of the workpiece, which is to be coated is located in a coating chamber in this arrangement, in which, in relation to the inside of the spray gun, a sub-atmospheric pressure is produced, for example a sub-atmospheric pressure of less than 100 mbar, while in the spray gun a pressure of approximately 1000 mbar prevails, in other words approximately ambient pressure. A broad coating beam can be produced by setting a pressure gradient of this kind between the inside of the spray gun and the coating chamber, by means of which the surface of the workpiece can be coated with a uniformity never previously attainable.
In this connection this basic principle has in the meantime been developed considerably further. EP 1 479 788 A1 shows for example a hybrid method which builds on the basic method of EP 0776 594 B1.
In this connection these methods are particularly suitable to apply different metallic or non-metallic coatings, in particular also ceramic, carbidic or nitridic layer components, in thin layers.
Modern technical demands however are moving toward the replacement of even the multi-stage galvanic or electrolytic method described above with reference to an exemplary process by thermal spraying methods, since the thermally sprayed layers can be applied principally in one method step and much more efficiently, in other words with clearly higher deposition rates, i.e. in a much shorter time.
In this connection one has hitherto been of the opinion that the classic thermal spraying methods, in particular the LPPS method outlined briefly above and also the cold gas spraying method, are fundamentally not suitable for coating with zinc and zinc compounds. The reason for this view is that zinc has an enormously high vapor pressure even at relatively low temperatures. Thus zinc for example already has a vapor pressure of approximately 1000 mbar at approximately 900° C., while aluminum only reaches roughly the same vapor pressure at approximately 2000° C. and Al2O3 only displays it at about 3000° C.
Thus one has assumed up to now that the LPPS method is not worth considering for the thermal spraying of zinc-containing layers, primarily, but not only, because it was presumed that due to its high vapor pressure the zinc already escapes in the coating beam to such a large extent that no useful zinc-containing layers can be manufactured by means of thermal spraying. In this connection not only is the high vapor pressure of the zinc as such regarded as a fundamental problem, but also the large difference in the vapor pressure from other materials, which can be sprayed at the same time as the zinc. If a different material is sprayed simultaneously together with zinc, which is for example separately sprayed simultaneously with zinc, but at a considerably lower vapor pressure, then it has to be feared that already the ratio of the composition of zinc to further spraying materials will massively alter the coating beam, so that the sprayed layers no longer have the desired composition and thus the necessary corrosion protection cannot be attained.
Accordingly, no suitable zinc-containing spray materials and consequently also no corresponding thermal spraying methods exist in the prior art.